Nuclear fuel assemblies for powering nuclear reactors generally consist of large numbers of fuel rods contained in discrete fuel rod assemblies. These assemblies or cells generally consist of a bottom end fitting or nozzle, a plurality of fuel rods extending upwardly therefrom and spaced from each other in a square pitch configuration, orientating or support grids spaced along the length of the assembly, a plurality of control guide tubes interspersed throughout the rod assembly, and a top end fitting or cap. The assembly is installed and removed from the reactor as a unit.
When the fuel rods have expended a large amount of their available energy, they are considered to be "spent" and the fuel rod assembly is pulled from the reactor and temporarily stored in an adjacent pool until the assemblies are transported to a reprocessing center or to permanent or temporary storage. Even though the rods are considered "spent" they are still highly radioactive and constitute a very real hazard both to personnel and to property.
In general, there are a number of alternatives available for disposition of the radioactive spent fuel rods, none of which is totally satisfactory. The fuel assemblies can be enclosed in a suitable basket and cask arrangement and shipped to a storage facility, or possibly, to a reprocessing plant. A second alternative is to store the spent fuel in a dry storage system. Dry storage entails either the use of a large number of metal casks or the building of massive concrete containers either above or below ground, which is a very expensive process, and, where the storage system is above ground, it is not readily acceptable to people living or working in its vicinity. A third alternative is the storage of the fuel units in the existing water pool originally designed for temporary storage. This type of storage is the simplest and cheapest, since the fuel rod assemblies can remain in the pool and be left there until the appropriate governmental agency collects them, often at the end of the life of the nuclear plant. However, such storage pools have a limited capacity, and, where they are adjacent to the reactor, necessitate the construction of a new pool when one becomes full.
Numerous attempts have been made to increase the capacity of a pool through a process known as fuel rod compaction or consolidation. This process, in brief, comprises removing the rods from the fuel rod assembly and placing them in a storage canister where they are placed in rows with minimal spacing. It is possible, with this process, to place the rods from two or more fuel assemblies into a single canister, thereby achieving approximately a 2:1 reduction in required pool volume, or, conversely, a 2:1 increase in pool storage capacity. However, successful consolidation has been an elusive goal for a number of reasons. Inasmuch as the pools are approximately forty feet deep, and inasmuch as the rods must remain immersed in the water at all times, all of the consolidation operations must be performed under the shield and cooling water. In addition, even though the rods are kept under water, the process could be quite hazardous to personnel performing the operation.
Prior art arrangements for achieving rod consolidation have included a system whereby the rods are pulled out row-by-row, as in, for example, a 14.times.14 matrix of rods, lifted and deposited in a tapered interim storage container, which tapers from a large area top opening to a bottom that has the area of a storage canister. After the intermediate container has the rods from approximately two fuel assemblies deposited therein, the intermediate container is placed over a storage canister, the bottom plate of the tapered container is lowered to cause the rods to slide into the storage canister. If the rods jam or stick, as they often do, they must be pushed from above the pool by operators using long rods. This last operation is made more difficult in that the rods develop on their outside surfaces what is referred to in the trade as "crud". When the fuel rods are pulled, this radioactive crud is scraped off and clouds the water making it difficult for the operators to see what they are doing and contaminating the pool. The method just described has proven to be quite slow and complicated, and can be hazardous to personnel.
An additional problem is caused in the close spacing of the rods, which are held in discrete locations in a fuel cell, as well as in the rod storage holder. this close spacing prevents a sequential pulling of adjacent rods out of the fuel cell and placing them in the storage holder because of the bulkiness of the end of the rod pulling and carrying tool. The tool cannot readily reach down to the top of a rod without hitting or being impeded by immediately adjacent rods.
In U.S. Pat. No. 4,731,219 of Beneck et al, there is shown an apparatus for compacting fuel rods which utilizes a tapered "quiver" to compact the rods. A grid structure is mounted on top of the quiver for guiding the rods into their desired positions. Elements of the grid structures are movable with respect to each other to achieve proper location of the rods being compacted. All of the rods are pulled from the fuel cell in two steps or operations, with a rotation of the pulling head after the first step to locate the multiple pullers over the rods that remained after the first pulling step. Various other prior art systems and method have been developed, none of which has proved to be wholly satisfactory.
Additionally, because of the depth of the pool and hence the extreme length of any pulling tool, precise positioning of the free or distal end of the tool over the rods is difficult to achieve, and, hence, precise location over a rod to be pulled becomes a hit or miss proposition. In U.S. Pat. No. 4,659,536 of Baudro, there is shown an indexing arrangement for a precise locating of the pulling tool. Baudro uses a guide funnel for guiding the pulling tool mounted on an indexing platform, which is movable along a pair of rails in a first direction. The rails are, in turn, mounted on a platform that is movable along a second pair of rails at right angles to the first pair of rails. Thus the funnel can be positioned over any rod in the array within the fuel cell, for example. Each of the indexing platforms requires its own drive means, which is responsive to control signals, as from a programmed computer. The overall indexing system is quite complicated and requires an indexing or locating step for each rod in the array of rods.